Field of the Invention
The present invention relates to a fibre optic based intrusion sensing system comprising at least one fiber optic cable buried in a shallow trench in the ground, a fibre optic interrogator measuring a predetermined property related to a change in the length of the cable and a control unit, wherein the cable having one end connected to an interrogator and a free second end at the end of the monitored perimeter, wherein the control unit is connected to the interrogator and adapted to analyse the measurements of said predetermined property and adapted to detect the presence of a specific object on the surface near the perimeter and identify the type of the object.
Description of Related Art
U.S. Pat. No. 5,194,847 A by Taylor and Lee discloses an apparatus for sensing intrusion into a predefined perimeter using buried fibre optic cables. The fiber optic cable is placed along a predefined perimeter which shall be monitored. The used apparatus roughly consists of a light source, an interferometer and a photodetector, which is able to detect the change of the backscattered light in an optical fiber. This change is interpreted as an intrusion. Using the time of flight of the light pulse in the fiber allows for locating the intrusion.
WO 2011/058312 A2 by Hill D. J. and McEwen-King M. describes a method for distributed sensing comprising a plurality of longitudinal sensing portions, which are all located along one cable. The different sensing portions are used to measure different sensing functions.
WO 2012/022934 A2 by McEwen-King M., Hill D. J. and Godfrey A. describes a system for the detection of moving objects based on distributed acoustic sensing along one buried fibre optic cable. The object is detected via the acoustic signal it produces during the movement over the fibre optic cable.
US 2012/130930 A1 by Klar et al. discloses a system, where one fibre optic cable is buried in a shallow trench and in deep boreholes to detect underground tunneling. The effect of tunnel excavation on the strains in the fiber optic cables is investigated. It is stated that surface loads may induce strains in a buried cable at a shallow depth which may disturb the signal caused by the tunneling. Tests with different surface loads were performed and the corresponding strain in one cable was measured. No attempt was made to identify the loads on the surface using the strain along the cable.
U.S. Pat. No. 4,482,890 A by Forbes G. et al. discloses an intruder detection system using a cable containing several parallel fibres wherein the fibres are placed in contact to each other. The cable with multiple fibres is used as a microbend sensor to measure compression of the soil due to an intruder.
Juarez J. C., Maier E. W., Choi K. N., and Taylor H. F. have published a paper “Distributed fiber-optic intrusion sensor system” in Journal of Lightwave Technology, vol. 23, pp. 2081-2087, 2005. Juarez J. C., Taylor H. F. have published a paper “Field test of a distributed fiber-optic intrusion sensor system for long perimeters”, Applied Optics, Vol. 46, Issue 11, pp. 1968-1971, 2007. Park J., Lee W. and Taylor H. F. have published a paper “A fiber optic intrusion sensor with the configuration of an optical time domain reflectometer using coherent interference of Rayleigh backscattering” in Proc. SPIE, 3555, 49-56, 1998. Juarez et al. as well as Park et al. at Texas A&M University improved the perimeter intrusion detection system based on U.S. Pat. No. 5,194,847 A. Klar A., Linker R. have also published a paper “Feasibility study of automated detection of tunnel excavation by Brillouin optical time domain reflectometry”, Tunneling and Underground Space Technology 25, 575-586, 2010. These authors also showed applicability of the system for intrusion detection and localization of the intruder in laboratory and field tests.
Madsen C. K., Bae T., Snider T. have published “Intruder Signature Analysis from a Phase-sensitive Distributed Fiber-optic Perimeter Sensor” in Fiber Optic Sensors and Applications V, Proc. of SPIE Vol. 6770, 67700K, 2007. A further publication is from Madsen C. K., Snider W. T., Atkins R. A., and Simcik J. C. as “Real-Time Processing of a Phase-Sensitive Distributed Fiber Optic Perimeter Sensor” in Proc. SPIE, Sensors, and Command, Control, Communications, and Intelligence (C3I) Technologies for Homeland Security and Homeland Defense VII, vol. 6943, no. 6943-36, 2008. These two publications, Madsen et al. (2007) and Madsen et al. (2008), show that these groups were able to extract a vibration signature of the intruder from the measured data by signal processing using the technology based on U.S. Pat. No. 5,194,847 A. This signature allows for a distinction between different intruders (e.g. a pedestrian and a car). The signature showed for a car, however, might be also caused by other machines or vehicles. Furthermore they managed to do the signal processing to provide the signature of the intruder in real-time.
Kirkendall C. K., Bartolo R., Salzano J., Daley K. have shown with their publication, “Distributed Fiber Optic Sensing for Homeland Security” in Naval Research Laboratory, Washington D.C., 2007, that they had developed their own distributed fiber optic sensing system to monitor intrusion of a perimeter and borders. In their system the fiber optic cable works as distributed seismic sensor. The intruder induces seismic waves in the subsoil which are detected along the cable. In field tests they show applicability to detection and localization of the intruder in the same way as the system used by Madsen et al. (2007).